The photocurrent action spectra or external quantum efficiency (EQE) measurement of semiconductor thin films is an important parameter for solar cell development. Values higher than 80% photon to electron conversion efficiency over a broad range of wavelengths are a necessary (but not sufficient) condition for the development of highly efficient solar cells. However, this measurement is usually performed on the actual solar cells, i.e., the knowledge occurs once the final device is assembled. It would be very desirable to gain such knowledge beforehand, by having a simple probe measurement of the photoactive semiconductor, as this would save cost and research time.